1. Field of the Invention
The present invention relates to a decay characteristic measuring apparatus for measuring the decay time and the like of a light beam, e.g., fluorescence generated upon light irradiation on an object having decay characteristics.
2. Related Background Art
Various systems are conventionally proposed for measurement of the decay characteristics of fluorescence or the like. These systems can be roughly classified into (1) a system for measuring the waveform of a change of light intensity to obtain a decay waveform from the measured waveform (to be referred to as a direct method hereinafter) and (2) a system using intensity-modulated excitation light to obtain decay characteristics from a phase difference with respect to decay signal light (to be referred to as a phase difference method hereinafter).
The following arrangements are available as typical specific arrangements of apparatuses using the conventional direct method:
1 Light Source+Photomultiplier (PMT)+Oscilloscope
This apparatus is used in a relatively slow region and rarely used in a high-speed region of 10 ns or less where a distortion of an electrical signal (e.g., ringing and overshoot) or a distortion of the detector itself (e.g., spread of transit time) poses a problem.
2 Short Pulse Laser+High-Speed PMT+Constant Fraction Discriminator+TAC (Time to Amplitude Converter)+PHA (Pulse Height Analyzer)
A time resolving power of several tens ps can be obtained by a time-correlated single photon counting method.
3 Streak Camera (Real Time Measurement)
A time resolving power in units of subpico seconds can be obtained. A single phenomenon can also be observed. In a system for overlapping (multiplying) a phenomenon many times in a single photoelectron region to obtain a waveform, a sufficient dynamic range can be obtained, which enables highly precise measurement of decay characteristics. Wavelength information can simultaneously be acquired using a spectroscope. This apparatus is excellent as a time-resolved spectroscope.
4 Optical Oscilloscope (Sampling Measurement)
This is a sampling type optical waveform observing apparatus using a streak tube. It is not suitable for measurement of extremely low intensity light because of a poor collection efficiency of target measurement light in this method, though the dynamic range in the measurement light amount region is excellent.
5 Boxcar (Sampling Measurement)
An electrical gate system is applied, and an inexpensive system is available. The time resolving power (.gtoreq.100 ps) is determined by the frequency response and electrical gate width of a detector.
6 Kerr cell (Optical Sampling Measurement)
An optical gate system is applied. The time resolving power depends on the laser pulse width for sampling, which can be shortened.
The phase difference method is suitable to obtain a decay time constant with a somewhat specified waveform at a high speed because of its high light utilization efficiency. A system or apparatus like an optical fiber thermometer usable in an environment with large disturbance noise such as a high electromagnetic field is proposed, in which a ruby or alexandrite crystal whose a fluorescence decay time largely changes in accordance with a temperature is used as a probe, and a phase difference between an excitation light beam and fluorescence is converted into a frequency by a PLL (Phase-Locked Loop), thereby measuring a lifetime (e.g., "Zhiyi Zhang et al., Rev. Sci. Instrum. 64 (9), pp. 2531-2540 (1993)", Japanese Patent Laid-Open No 63-308596, "T. Bosselmen et al., Proc. 2nd Optical Fiber Sensor Conf., 1984, Stuttgart, pp 151-154", "Zhiyi Zhang et al., Rev. Sci. Instrum. 62, p. 1735 (1991)"). If the waveform is not specified, or multicomponent decay characteristics are to be measured, the modulation frequency is changed (when the excitation light is modulated by sine wave modulation), or the target frequency (an integer multiple of a repetition period when the excitation light is an impulse) is changed. Data of phase difference and intensity are acquired every time the frequency is changed, and processing such as fitting is performed, thereby obtaining the decay characteristics (J. R. Lakowicz et al., SPIE vol. 1204 Time-Resolved Laser Spectroscopy in Biochemistry II, pp. 13-20 (1990)).